In a spin valve magnetoresistive head, a spin valve magnetoresistive sensor is sandwiched in top and bottom read gap layers, and is contacted by longitudinal bias and conductor layers in the end regions of the sensor. The sensor detects magnetic field signals through the resistance changes of the sensor as a function of the strength of the magnetic flux being sensed by the sensor. A pronounced magnetoresistance, called giant magnetoresistance (GMR), is employed in spin valve magnetoresistive sensors, the essential feature being at least two ferromagnetic films separated by a nonferromagnetic spacer layer.
A spin valve sensor may comprise a sandwich structure comprising two ferromagnetic films separated by a nonmagnetic spacer layer in which the magnetization of one of the ferromagnetic films (called a reference layer) is "pinned". The pinning may be achieved by either depositing the reference layer on top of an antiferromagnetic film or by depositing the antiferromagnetic film on top of the reference layer in the presence of a magnetic field oriented in a first direction, typically called the "transverse" direction. The sensor detects magnetic field signals through the resistance changes of the sensor as a function of the strength of the magnetic flux being sensed by the sensor. The spin valve sensor is typically employed to read data recorded as magnetic field signals on a recording surface of a recording medium. Examples of recording media include magnetic disks which rotate at high speed, or magnetic tapes which are moved in a linear fashion. The spin valve sensor is typically closely spaced from the recording surface, often on an air bearing slider which has an air bearing surface which rides on an air bearing at the surface of a rotating disk, but also as a contact or near contact transducer. The "transverse direction" in a sensor is typically perpendicular to the recording surface and perpendicular to the air bearing surface of the slider.
The ferromagnetic sense layer may also have its magnetization in the end regions (those portions of the sense layer on either side of the read region) pinned by another antiferromagnetic film or a hard magnetic film for sensor stability. To ensure this pinning, a high unidirectional anisotropy field (HU.sub.UA), induced by exchange coupling between the ferromagnetic and antiferromagnetic films, is needed in the antiferromagnetic stabilization scheme, while a high coercivity (H.sub.c) of the hard magnetic film is needed in the hard magnetic stabilization scheme. The magnetization of the ferromagnetic sense layer in the end regions must be at an angle approximating the perpendicular to the magnetization of the reference layer, and preferably perpendicular to the magnetization of the pinned layer such that the magnetization is parallel to the air bearing surface or to the recording surface (called the longitudinal direction). Only the magnetization of the ferromagnetic sense layer in the read region is not rigidly pinned by an antiferromagnetic film or a hard-magnetic film. However, three magnetic fields coexist in the ferromagnetic sense layer. One is a demagnetizing field (H.sub.D) produced by magnetostatic coupling between the sense and reference layers, one is a ferromagnetic/ferromagnetic coupling field (H.sub.F) across the spacer layer, and the other is a sense-current-induced field (H.sub.I). With an optimal design, H.sub.D is balanced by the sum of H.sub.F and H.sub.I, so that the magnetization of the sense layer is oriented in the longitudinal direction. In the presence of an external magnetic field, the magnetization of the sense layer rotates and this rotation changes the resistance characteristics of the sensor due to GMR effects.
As described in coassigned U.S. Pat. No. 5,528,440, Fontana et al., issued Jun. 18, 1996, the magnetization of the ferromagnetic film in the end regions may also be pinned by exchange coupling to an antiferromagnetic film. In order to pin the end region magnetization in a direction perpendicular to the magnetization of the reference layer, different antiferromagnetic materials are used. Specifically, the reference layer in the spin valve sensor is pinned by exchange coupling to an iron-manganese (Fe--Mn) film. An antiferromagnetic nickel-manganese (NiMn) film is used in the end regions. Exchange-coupled Ni--Fe/Fe--Mn films in the read region have a blocking temperature (where exchange coupling disappears) of 150.degree. C., substantially lower than that of exchange-coupled Ni--Fe/Ni--Mn films (&gt;450.degree. C.) in the end regions.
In fabrication, after all of the films have been deposited, the sensor is placed in an annealing oven and heated to a temperature of approximately 240.degree. C. in the presence of an applied magnetic field in the longitudinal direction. When heated, the Ni--Mn film becomes antiferromagnetic and, after cooling in the presence of a magnetic field, the magnetization of the ferromagnetic film in the end regions becomes pinned by the Ni--Mn film. Then, the sensor is again heated in the presence of a magnetic field, perpendicular to the previous magnetic field direction. Heating the sensor to approximately 180.degree. C. and cooling in the presence of the magnetic field allows the magnetization of the reference layer to be pinned by the Fe--Mn film in the transverse direction.
However, two problems have prevented practical application of the Fe--Mn film in the spin valve magnetoresistive sensor, despite their strong antiferromagnetism. One problem is the low resistance to corrosion exhibited by the Fe--Mn film which causes great difficulty in implementing it into the fabrication process. Another problem is the low blocking temperature of the Ni--Fe/Fe--Mn films. The magnetization of the reference layer will be canted during operation of the sensor at a drive temperature (.about.120.degree. C.).
Hence, most practical spin valve sensors employ an antiferromagnetic film in the read region for sensor operation and a hard-magnetic film in the end regions for sensor stability. The use of this hard-magnetic stabilization scheme for the 1 Gb/in2 spin valve sensor with a 8 nm thick sense layer leads to good read performance, such as quiet GMR responses, high read sensitivity and effective side reading suppression. To perform magnetic recording beyond 1 Gb/in2, the sense layer thickness must be reduced, and the hard magnetic film thickness must be correspondingly reduced for optimal moment matching to attain sensor stability and yet retain high signal sensitivity. Since H.sub.c decreases as the hard magnetic film thickness decreases, GMR responses may show hysteretic noises. In addition, stray fields of the hard magnetic film cause the edges of the spin valve sensor to be ineffective in reading signals, and the size of this ineffective zone (.gtoreq.0.1 .mu.m in each edge) does not decrease as the submicron width of the sensor further decreases for a high track density. Hence, the hard magnetic stabilization scheme may not be viable for the multigigabit recording density. In contrast, since HU.sub.UA increases as the ferromagnetic film thickness decreases, the antiferromagnetic stabilization scheme is preferred for the multigigabit magnetic recording.